1. Field of the Invention
The present invention relates to a technique for quantizing input image data having gradation of an m value (m is a natural number) into output image data having gradation of an n value (n is a natural number smaller than m).
2. Description of the Related Art
Information output apparatuses for digital cameras, personal computers, and the like include various types of recording apparatuses for performing recording of information such as desired characters, images, or the like on a sheet recording medium such as recording paper, film, or the like. In such recording apparatuses, methods for forming characters or an image on a recording medium by adhering a recording material on the recording medium have been put to practical use. An example of such methods is inkjet recording apparatuses.
Generally, in order to increase recording speeds and image quality, an inkjet recording apparatus includes a nozzle group formed by integrating and arranging arrays of ink discharge ports (nozzles) that can discharge ink of the same color and the same density. Further, in a case where the inkjet recording apparatus includes inks of different colors, a nozzle group is provided for each color. Further, as to nozzles that can discharge ink of the same color and different densities or ink of the same color and the same density by changing the amounts of discharge in several steps, a nozzle group can be provided respectively.
In such image forming apparatuses, image processing methods for converting multi-valued input image data into print data corresponding to a record signal of dots includes an error diffusion method. In the error diffusion method, by diffusing an error produced in pixels in binarization processing in peripheral pixels to be processed later, pseudo-gradation expression can be performed.
When an image is formed by the image forming apparatus using image data on which gradation number conversion is performed, a multipass recording method for forming the image by a plurality of passes (scans) is often used. The method for calculating print data in each pass includes a method that uses a mask pattern. Japanese Patent Application Laid-Open No. 2002-096455 discusses an example of the method.
Binary image data is separated into each pass by an AND operation with a mask pattern for each pass. By the above-described multipass recording method, effects due to variation in the ink discharge amounts and discharge directions are reduced by half. Accordingly, the density unevenness in the formed image is reduced.
In the multipass recording method using the mask pattern, the data obtained by integrating the print data of each pass is equal to the original binary image data. However, if a relative positional deviation occurs between each pass, the graininess of the image deteriorates, and the image quality of the output image significantly decreases.
The deterioration in the image quality in the multipass recording method using the mask pattern is due to low dispersiveness of the dots printed in each pass, which also causes decrease in dispersiveness in the dot arrangement in the output image, when the positional deviation has occurred.
Further, if the above-mentioned problem is generalized, a common problem exists also in the intercolor exclusive technique and small, medium, and large dot exclusive technique. According to the intercolor exclusive technique, in binarization processing in an apparatus that has ink of a plurality of different colors, dot arrangement is controlled so that the overlap of the dots of the different colors is reduced. In the intercolor exclusive technique, while the dispersiveness in the dot arrangement of each color is kept at a high level, the dispersiveness in the dot arrangement in the image in which each color is integrated is also to be kept at a high level.
According to the small, medium, and large dot exclusive technique, in an apparatus that can discharge ink of the same density by changing the discharge amounts at several levels, the dispersiveness in the dot arrangement of different discharge amounts is increased. In the small, medium, and large dot exclusive technique, while the dispersiveness in the dot arrangement of each discharge amount is kept at a high level, the dispersiveness in the image in which the dots of each discharge amount is integrated is also to be kept at a high level.
As described above, in the image forming apparatus that can output different types of dots, while the dispersiveness of the various types of dots is increased, the dispersiveness of the dots that are formed by integrating the various types of dots is also to be increased. By the processing, the graininess and the density unevenness of the formed image can be improved.
In the description, the different types of dots include dots in each pass in the multipass recording method, dots of ink of each color in the multicolor recording, and dots of each discharge amount in the small, medium, and large dot recording.
Japanese Patent Application Laid-Open No. 2008-258866 discusses an interpass exclusive technique in the multipass recording. That is, Japanese Patent Application Laid-Open No. 2008-258866 discusses a technique for detecting a relative positional deviation amount between passes. If the positional deviation amount is small, an error diffusion method is used, and if the positional deviation amount is large, a dither matrix that is strongly designed to the deviation is used. Methods for generating a dither matrix include a method discussed in Japanese Patent Application Laid-Open No. 2007-106097. In the method, optimization calculation including the graininess of each pass in an evaluation value is performed.
However, the technique discussed in Japanese Patent Application Laid-Open No. 2008-258866 requires means for detecting the positional deviation amount, and therefore, the cost of the apparatus becomes high. Further, if the positional deviation amount is large, only the gradation conversion using the dither matrix can be used. The gradation conversion using the dither matrix is faster than that using the error diffusion method, however, it is known that the gradation conversion using the dither matrix is rather inferior to that using the error diffusion method in the graininess.
To solve the problems, a technique for increasing the dispersiveness of the dots that are formed by integrating each pass while increasing the dispersiveness of the dots of each pass without limiting the gradation conversion means is desired.